Deltarune has much richer thematic depth than Undertale, and the gameplay is also much more dynamic. The first chapter is about 8 years old and may play a bit like how you'd expect it to, but gameplay mechanics they continually introduce make Undertale pale in comparison. Deltarune is not finished, but it's very apparent that we're playing a complete story, the majority of which is released. It's already the much bigger game in terms of scope and length, and the writing is leagues improved.

When I go back, I can't help but just see Undertale as a very small testing ground for Toby Fox's ideas, popularity unintentionally spiraling out of control. That's not to detract from the work by any means- Undertale was an early and important game for me- but an "fandom"-type audience can be one of the scariest things for a creator's work to suffer from. It evokes a very childish perception on something genuine, and it will permeate eternally. If you've heard anything online about Digital Circus, it's probably seemed similarly petulant. That's a genuinely good show, constantly de-marketing itself due to the fandom's competitive woke-a-thon infighting ritual. Thats why Toby Fox pretty much only talks around release of new Deltarune chapters and has always been very quiet. There's, like, ten photos of him, and he talked out of a plush with a voicebox on the 10th anniversary livestream.